Re: [RFC PATCH v1 36/57] xen: Remove PAGE_SIZE compile-time constant assumption

[Ryan Roberts <ryan.roberts@xxxxxxx>]

+ Juergen Gross, Stefano Stabellini

This was a rather tricky series to get the recipients correct for and my script
did not realize that "supporter" was a pseudonym for "maintainer" so you were
missed off the original post. Appologies!

More context in cover letter:
https://lore.kernel.org/all/20241014105514.3206191-1-ryan.roberts@xxxxxxx/


On 14/10/2024 11:58, Ryan Roberts wrote:
> To prepare for supporting boot-time page size selection, refactor code
> to remove assumptions about PAGE_SIZE being compile-time constant. Code
> intended to be equivalent when compile-time page size is active.
> 
> Allocate enough "frame_list" static storage in the balloon driver for
> the maximum supported page size. Although continue to use only the first
> PAGE_SIZE of the buffer at run-time to maintain existing behaviour.
> 
> Refactor xen_biovec_phys_mergeable() to convert ifdeffery to c if/else.
> For compile-time page size, the compiler will choose one branch and
> strip the dead one. For boot-time, it can be evaluated at run time.
> 
> Refactor a BUILD_BUG_ON to evaluate the limit (when the minimum
> supported page size is selected at boot-time).
> 
> Reserve enough storage for max page size in "struct remap_data" and
> "struct xenbus_map_node".
> 
> Signed-off-by: Ryan Roberts <ryan.roberts@xxxxxxx>
> ---
> 
> ***NOTE***
> Any confused maintainers may want to read the cover note here for context:
> https://lore.kernel.org/all/20241014105514.3206191-1-ryan.roberts@xxxxxxx/
> 
>  drivers/xen/balloon.c              | 11 ++++++-----
>  drivers/xen/biomerge.c             | 12 ++++++------
>  drivers/xen/privcmd.c              |  2 +-
>  drivers/xen/xenbus/xenbus_client.c |  5 +++--
>  drivers/xen/xlate_mmu.c            |  6 +++---
>  include/xen/page.h                 |  2 ++
>  6 files changed, 21 insertions(+), 17 deletions(-)
> 
> diff --git a/drivers/xen/balloon.c b/drivers/xen/balloon.c
> index 528395133b4f8..0ed5f6453af0e 100644
> --- a/drivers/xen/balloon.c
> +++ b/drivers/xen/balloon.c
> @@ -131,7 +131,8 @@ struct balloon_stats balloon_stats;
>  EXPORT_SYMBOL_GPL(balloon_stats);
>  
>  /* We increase/decrease in batches which fit in a page */
> -static xen_pfn_t frame_list[PAGE_SIZE / sizeof(xen_pfn_t)];
> +static xen_pfn_t frame_list[PAGE_SIZE_MAX / sizeof(xen_pfn_t)];
> +#define FRAME_LIST_NR_ENTRIES (PAGE_SIZE / sizeof(xen_pfn_t))
>  
>  
>  /* List of ballooned pages, threaded through the mem_map array. */
> @@ -389,8 +390,8 @@ static enum bp_state increase_reservation(unsigned long 
> nr_pages)
>       unsigned long i;
>       struct page   *page;
>  
> -     if (nr_pages > ARRAY_SIZE(frame_list))
> -             nr_pages = ARRAY_SIZE(frame_list);
> +     if (nr_pages > FRAME_LIST_NR_ENTRIES)
> +             nr_pages = FRAME_LIST_NR_ENTRIES;
>  
>       page = list_first_entry_or_null(&ballooned_pages, struct page, lru);
>       for (i = 0; i < nr_pages; i++) {
> @@ -434,8 +435,8 @@ static enum bp_state decrease_reservation(unsigned long 
> nr_pages, gfp_t gfp)
>       int ret;
>       LIST_HEAD(pages);
>  
> -     if (nr_pages > ARRAY_SIZE(frame_list))
> -             nr_pages = ARRAY_SIZE(frame_list);
> +     if (nr_pages > FRAME_LIST_NR_ENTRIES)
> +             nr_pages = FRAME_LIST_NR_ENTRIES;
>  
>       for (i = 0; i < nr_pages; i++) {
>               page = alloc_page(gfp);
> diff --git a/drivers/xen/biomerge.c b/drivers/xen/biomerge.c
> index 05a286d24f148..28f0887e40026 100644
> --- a/drivers/xen/biomerge.c
> +++ b/drivers/xen/biomerge.c
> @@ -8,16 +8,16 @@
>  bool xen_biovec_phys_mergeable(const struct bio_vec *vec1,
>                              const struct page *page)
>  {
> -#if XEN_PAGE_SIZE == PAGE_SIZE
> -     unsigned long bfn1 = pfn_to_bfn(page_to_pfn(vec1->bv_page));
> -     unsigned long bfn2 = pfn_to_bfn(page_to_pfn(page));
> +     if (XEN_PAGE_SIZE == PAGE_SIZE) {
> +             unsigned long bfn1 = pfn_to_bfn(page_to_pfn(vec1->bv_page));
> +             unsigned long bfn2 = pfn_to_bfn(page_to_pfn(page));
> +
> +             return bfn1 + PFN_DOWN(vec1->bv_offset + vec1->bv_len) == bfn2;
> +     }
>  
> -     return bfn1 + PFN_DOWN(vec1->bv_offset + vec1->bv_len) == bfn2;
> -#else
>       /*
>        * XXX: Add support for merging bio_vec when using different page
>        * size in Xen and Linux.
>        */
>       return false;
> -#endif
>  }
> diff --git a/drivers/xen/privcmd.c b/drivers/xen/privcmd.c
> index 9563650dfbafc..847f7b806caf7 100644
> --- a/drivers/xen/privcmd.c
> +++ b/drivers/xen/privcmd.c
> @@ -557,7 +557,7 @@ static long privcmd_ioctl_mmap_batch(
>       state.global_error  = 0;
>       state.version       = version;
>  
> -     BUILD_BUG_ON(((PAGE_SIZE / sizeof(xen_pfn_t)) % XEN_PFN_PER_PAGE) != 0);
> +     BUILD_BUG_ON(((PAGE_SIZE_MIN / sizeof(xen_pfn_t)) % 
> XEN_PFN_PER_PAGE_MAX) != 0);
>       /* mmap_batch_fn guarantees ret == 0 */
>       BUG_ON(traverse_pages_block(m.num, sizeof(xen_pfn_t),
>                                   &pagelist, mmap_batch_fn, &state));
> diff --git a/drivers/xen/xenbus/xenbus_client.c 
> b/drivers/xen/xenbus/xenbus_client.c
> index 51b3124b0d56c..99bde836c10c4 100644
> --- a/drivers/xen/xenbus/xenbus_client.c
> +++ b/drivers/xen/xenbus/xenbus_client.c
> @@ -49,9 +49,10 @@
>  
>  #include "xenbus.h"
>  
> -#define XENBUS_PAGES(_grants)        (DIV_ROUND_UP(_grants, 
> XEN_PFN_PER_PAGE))
> +#define XENBUS_PAGES(_grants)                (DIV_ROUND_UP(_grants, 
> XEN_PFN_PER_PAGE))
> +#define XENBUS_PAGES_MAX(_grants)    (DIV_ROUND_UP(_grants, 
> XEN_PFN_PER_PAGE_MIN))
>  
> -#define XENBUS_MAX_RING_PAGES        (XENBUS_PAGES(XENBUS_MAX_RING_GRANTS))
> +#define XENBUS_MAX_RING_PAGES                
> (XENBUS_PAGES_MAX(XENBUS_MAX_RING_GRANTS))
>  
>  struct xenbus_map_node {
>       struct list_head next;
> diff --git a/drivers/xen/xlate_mmu.c b/drivers/xen/xlate_mmu.c
> index f17c4c03db30c..a757c801a7542 100644
> --- a/drivers/xen/xlate_mmu.c
> +++ b/drivers/xen/xlate_mmu.c
> @@ -74,9 +74,9 @@ struct remap_data {
>       int mapped;
>  
>       /* Hypercall parameters */
> -     int h_errs[XEN_PFN_PER_PAGE];
> -     xen_ulong_t h_idxs[XEN_PFN_PER_PAGE];
> -     xen_pfn_t h_gpfns[XEN_PFN_PER_PAGE];
> +     int h_errs[XEN_PFN_PER_PAGE_MAX];
> +     xen_ulong_t h_idxs[XEN_PFN_PER_PAGE_MAX];
> +     xen_pfn_t h_gpfns[XEN_PFN_PER_PAGE_MAX];
>  
>       int h_iter;     /* Iterator */
>  };
> diff --git a/include/xen/page.h b/include/xen/page.h
> index 285677b42943a..86683a30038a3 100644
> --- a/include/xen/page.h
> +++ b/include/xen/page.h
> @@ -21,6 +21,8 @@
>       ((page_to_pfn(page)) << (PAGE_SHIFT - XEN_PAGE_SHIFT))
>  
>  #define XEN_PFN_PER_PAGE     (PAGE_SIZE / XEN_PAGE_SIZE)
> +#define XEN_PFN_PER_PAGE_MIN (PAGE_SIZE_MIN / XEN_PAGE_SIZE)
> +#define XEN_PFN_PER_PAGE_MAX (PAGE_SIZE_MAX / XEN_PAGE_SIZE)
>  
>  #define XEN_PFN_DOWN(x)      ((x) >> XEN_PAGE_SHIFT)
>  #define XEN_PFN_UP(x)        (((x) + XEN_PAGE_SIZE-1) >> XEN_PAGE_SHIFT)